Power conversion system for reducing dc-link capacitor stress
A power conversion system includes a first direct current to alternating current (DC-AC) inverter, a second DC-AC inverter, an inverter controller, a direct current (DC)-link circuit connecting the first DC-AC inverter and the second DC-AC inverter, a neutral-point balancer (NPB), an NPB circuit, and an NPB controller. The inverter controller is configured to provide a pulse-width modulation (PWM), signal to the first DC-AC inverter and the second DC-AC inverter. The NPB circuit is configured to reduce low frequency current oscillations in the DC-link circuit by switching between a first state for transferring charge from a positive voltage line to a neutral voltage line, and a second state for transferring charge from the neutral voltage line to a negative voltage line. The NPB controller is configured to control the switching of the NPB circuit and is synchronized with the PWM signal from the inverter controller.
1 . A power conversion system comprising:
a first direct current (DC)-alternating current (AC) inverter;
a second DC-AC inverter;
an inverter controller coupled to the first DC-AC inverter and the second first DC-AC inverter and configured to provide a pulse-width modulation, (PWM) signal to the first DC-AC inverter and the second DC-AC inverter;
a DC-link circuit coupling the first DC-AC inverter and the second DC-AC inverter in parallel, the DC link circuit and comprising:
a positive voltage line;
a negative voltage line;
a neutral voltage line;
a high-frequency positive-line capacitor;
a low-frequency positive-line capacitor, wherein each of the high-frequency positive-line capacitor and the low-frequency positive-line capacitor couples the neutral voltage line with the positive voltage line;
a high-frequency negative-line capacitor; and
a low-frequency negative-line capacitor, wherein each high-frequency negative-line capacitor and the low-frequency positive-line capacitor couples the neutral voltage line with the negative voltage line;
a neutral-point balancer (NPB) circuit coupled to the DC-link circuit and configured to reduce low-frequency current oscillations in the DC-link circuit by switching between a first state for transferring charge from the positive voltage line to the neutral voltage line, and a second state for transferring charge from the neutral voltage line to the negative voltage line; and
an NPB controller coupled to the NPB circuit, synchronized with the PWM signal, and configured to control the switching of the NPB circuit.
2 . The power conversion system of claim 1 , wherein the NPB circuit is a bidirectional DC-DC converter.
3 . The power conversion system of claim 2 , wherein the NPB circuit comprises:
a first antiparallel diode;
a positive line switch coupling the neutral voltage line with the positive voltage line using the first antiparallel diode;
a second antiparallel diode;
a negative line switch coupling the neutral voltage line with the negative voltage line using the second antiparallel diode; and
one or more inductors coupled to a coupling point between the first antiparallel diode and the second antiparallel diode, and to the neutral voltage line.
4 . The power conversion system of claim 1 , wherein each of the first DC-AC inverter and the second DC-AC inverter is a three-level inverter.
5 . The power conversion system of claim 1 , wherein the inverter controller is further configured to set an inverter switching frequency, and wherein the NPB controller is further configured to set an NPB circuit switching frequency of the NPB circuit to an integer multiple of the inverter switching frequency.
6 . The power conversion system of claim 1 , wherein the NPB controller is further configured to:
determine an optimal value for a phase angle between a carrier signal of the NPB circuit and the PWM signal at which high-frequency current oscillations in the DC-link circuit are below a threshold amplitude; and
adjust a phase of the carrier signal to make the phase angle approach the optimal value.
7 . The power conversion system of claim 6 , wherein the NPB controller is further configured to determine the optimal value from a look-up table based on a ratio between a switching frequency of the NPB circuit and an inverter switching frequency of the inverter controller, and further based on a modulation used by the inverter controller.
8 . The power conversion system of claim 6 , wherein the NPB controller is further configured to determine the optimal value by monitoring a root mean square (RMS) value of a high-frequency current through the high-frequency positive-line capacitor and the high-frequency negative-line capacitor and adjusting the phase to set a value of the phase angle at which the RMS value is below the threshold amplitude.
9 . The power conversion system of claim 1 , wherein the inverter controller is further configured to set an inverter switching frequency, and wherein the high-frequency positive-line capacitor and the high-frequency negative-line capacitor are tuned to have a resonant frequency at a high-frequency that is a predefined frequency distance away from the inverter switching frequency.
10 . The power conversion system of claim 1 , wherein the high-frequency positive-line capacitor and the high-frequency negative-line capacitor are film type capacitors.
11 . The power conversion system of claim 1 , wherein the low-frequency positive-line capacitor and the low-frequency negative-line capacitor are configured to be tuned to make a branch coupling the low-frequency positive-line capacitor and the low-frequency negative-line capacitor have a resonant frequency in a frequency range between three times an output line frequency of the first DC-AC inverter and the second DC-AC inverter and a switching frequency of the first DC-AC inverter and the second DC-AC inverter.
12 . The power conversion system of claim 11 , wherein the resonant frequency is a predefined frequency distance away from boundaries of the frequency range.
13 . The power conversion system of claim 1 , wherein the low-frequency positive-line capacitor and the low-frequency negative-line capacitor are electrolytic type capacitors.
14 . The power conversion system of claim 2 , wherein each of the first DC-AC inverter and the second DC-AC inverter is a three-level inverter.
15 . The power conversion system of claim 2 , wherein the inverter controller is further configured to set an inverter switching frequency, and wherein the NPB controller is further configured to set a switching frequency of the NPB circuit to an integer multiple of the inverter switching frequency.
16 . The power conversion system of claim 2 , wherein the NPB controller is further configured to:
determine an optimal value for a phase angle between a carrier signal of the NPB circuit and the PWM signal at which high-frequency current oscillations in the DC-link circuit are below a threshold amplitude; and
adjust a phase of the carrier signal to make the phase angle approach the optimal value.
17 . The power conversion system of claim 2 , wherein the inverter controller is further configured to set an inverter switching frequency, and wherein the high-frequency positive-line capacitor and the high-frequency negative-line capacitor are configured to have a resonant frequency that is a predefined frequency distance away from the inverter switching frequency.
18 . The power conversion system of claim 2 , wherein the high-frequency positive-line capacitor and the high-frequency negative-line capacitor are film type capacitors.
19 . The power conversion system of claim 2 , wherein the low-frequency positive-line capacitor and the low-frequency negative-line capacitor are configured to make a branch coupling the low-frequency positive-line capacitor and the low-frequency negative-line capacitor have a resonant frequency in a frequency range between three times an output line frequency of the first DC-AC inverter and the second DC-AC inverter and a switching frequency of the first DC-AC inverter and the second DC-AC inverter.
20 . The power conversion system of claim 2 , wherein the low-frequency positive-line capacitor and the low-frequency negative-line capacitor are electrolytic type capacitors.